The invention relates to heterostructure field effect transistors with high modulation effectivity, of the type with having a heterostructure layer sequence deposited on the semiconductor substrate.
The new fields of application for modern communication electronics require the use of ever higher frequency ranges. In turn, this leads consistently to new requirements for the integrated circuits that form the basic elements of modern systems. Consequently, there is a demand for developing new types of transistors with low noise and improved power amplification or gain at ever higher operating frequencies. A completely new type of transistor was introduced for the first time by Mimura et al. "A New Field-effect Transistor With Selectively Doped GaAs/n-Al(x)Ga(1-x) As Heterojunctions" in Japanese Journal of Applied Physics, Vol. 19(5), pp. 225-27, 1980, with the development of an AlGaAs/GaAs heterostructure field-effect transistor (HFET). Thus, an experimental HFET concept with modulation doping--the term MODFET is also used frequently in this case--was presented by Dingle et al. "Electron Mobilities In Modulation-doped Semiconductor Heterostructure Superlattices", Applied Physics Letter, vol. 33(7), pp. 665-67, 1987, in this case for the production of a new type of high-speed transistor. Of importance for this is the purposeful use of a quantum effect occurring at a heterojunction in the active channel region for the charge carrier transport, in a so-called two-dimensional electron gas (2DEG), and the considerably improved transport qualities as compared to the pure semiconductor crystal, resulting from the spatial separation of free charge carriers in the channel region and caused by the reduction in the Coulomb scattering processes as a result of locally fixed, ionized impurity trunks in the doping region of a supply layer, in particular the very high mobility values with simultaneously high charge carrier concentrations.
The above mentioned type of transistor, formed on the basis of the semiconductor materials silicon (Si) and silicon/germanium alloys (Si/Ge) is known as p-conducting type from the article by T. P. Pearsall et al., "Ge.sub.x Si.sub.1-x Modulation-doped P-channel Field-effect Transistor, Proceedings of the 1.sup.st International Symposium On Silicon MBE, Vol. 85-7, pp. 400-405, May 1985, Toronto, Canada, Ed. J. C.Bean, Electrochem. Soc., Pennington, N.J.). The MODFET specified therein consists of a semiconductor layer sequence, composed of alternating layers of silicon and silicon/germanium alloys that permit the configuration of a p-MODFET with a p-conducting channel. This channel is also very narrowly defined through a targeted influencing of the band structure in the semiconductor material and is referred to as a so-called two-dimensional hole gas (2DHG). N-conducting MODFET's, formed on the basis of the same Si and Si/Ge semiconductor material U.S. Pat. No. 4,710,788; EP 02 28 516 B1; DE 37 31 000 C2 and DE 41 01 167 A1) are known as well. The references U.S. Pat. No. 4,710,788 and EP 02 28 516 B1 provide that a two-dimensional electron gas (2DEG) is generated with the aid of a heterojunction of a Si.sub.x Ge.sub.1-x /Si semiconductor layer sequence. In this case, the electrons are essentially conducted inside the Si semiconductor layer.
The references DE 37 31 000 C2, DE 41 01 167 A1 and EP 06 83 522 A2 show possible uses and varied designs for integrated semiconductor arrangements, composed of p-channel and n-channel HFET's, including the sequence of the individual semiconductor layers.
Also contained in the reference U.S. Pat. No. 4,710,788 are heterolayer sequences with several 2DEG channels, but which always have a highly doped region between neighboring channels. It is exactly these superimposed, highly doped regions, which screen each other mutually, so that only a limited control of the conducting channels through the gate is still possible. If the transistor is designed with one channel only, this screening effect does not occur, but the conductance is limited to only one zone of the two-dimensional charge carrier gas. Also, there is a natural limit on the number of charge carriers n.sub.s in a channel, and thus also a limit to the improvement of transistor characteristics. A value that determines the transistor characteristics is the so-called modulation effectivity (Foisy, M. C. et al., IEEE Translations on Electronic Development, Vol. 35, No. 7, pp. 871-77, 1988), which is a measure for the charge carriers that are present with high mobility in the two-dimensional charge carrier gas. Important transistor characteristics such as maximum operating frequency, transconductance, and transit frequency are positively influenced by an increase in the modulation effectivity.
It is thus the object of the present invention to provide a generic-type heterostructure field-effect transistor (n-HFET or p-HFET) with a heterostructure layer sequence, which on the whole results in an improvement of the transistor characteristics as compared to the state of the technology.